In one class of hydraulically-actuated fuel injectors, such as the type described in U.S. Pat. No. 5,460,329 to Sturman, an intensifier piston is hydraulically driven downward during an injection event by high pressure fluid acting on its top surface, and is retracted between injection events by high pressure actuation fluid acting on the underside of the piston. The piston may be driven using any suitable pressurized actuation fluid including possibly fuel fluid, but preferably uses pressurized engine lubricating oil. The piston is attached to move a relatively small diameter plunger that pressurizes fuel in a pump chamber. Because the area ratio of the piston to the plunger is often on the order of about 10 to 1, the fuel under the plunger can be raised to injection pressures that are about ten times as high as the actuation fluid pressure acting on the intensifier piston.
Engineers have come to recognize that undesirable engine noise and emissions can be improved over most portions of an engine's operating range when each injection event ends as abruptly as possible. Thus, there is a motivation to close the needle valve member that controls the nozzle outlet of the fuel injector as quickly as possible at the end of an injection event. In most instances, the needle valve member operates as a simple check valve, in that it is biased to a closed position with a spring but may be hydraulically lifted to an open position when fuel pressure is above some threshold capable of overcoming the biasing spring. In order to close such a needle valve as quickly as possible, it is necessary for fuel pressure to drop as quickly as possible at the end of an injection event so that the needle can move to close the nozzle outlet under the action of its biasing spring as quickly as possible without resistance from residual fuel pressure. Fuel pressure can be quickly dropped by abruptly ceasing the downward movement of the plunger and piston at the end of an injection event. Due at least in part to finite constraints relating to the electronic control valve that controls the flow of high pressure actuation fluid to the piston, there remains room for improving the performance of the Sturman fuel injector.
The present invention is directed to these and other problems associated with improving the performance of hydraulically-actuated fuel injectors having double acting intensifier pistons.